1. Field of the Invention
The present invention relates to shock waveform synthesis methods which are used for a shock response spectrum test, and particularly, to shock waveform synthesis methods for a shock response spectrum over a short time interval.
2. Description of the Conventional Art
Shock tests in environmental test are performed to evaluate an influence on a physical or functional performance of a test specimen to be exposed to shocks in its lifetime. The shock tests may be classified into classical shock tests such as half-sine, sawtooth, or the like specifying types of waveforms, and shock response spectrum tests which do not specify the types of waveforms and can advantageously achieve uniform results therefrom.
A shock response spectrum (hereinafter SRS) is a plot of the peak responses of a series of single degree of freedom (hereinafter SDOF) systems to an input transient. Here, the peak response indicates a maximax in the response history. There may theoretically be many types of waveforms satisfying the shock response spectrum which is required for specifications of the shock response spectrum test. The shock response spectrum test uses a shaker.
FIG. 1 is a schematic block diagram of a shaker system for performing the shock response spectrum test.
Referring to FIG. 1, the shaker system comprises a control system 1 and a power amplifier 2. A fixture 5 is placed between the shaker 3 and a test specimen 6, and an accelerometer 4 is mounted at a position at which the test specimen 6 is coupled to the fixture 5.
In general, mechanical shock applied to the test specimen 6 will cause the test specimen 6 to respond to (a) forced frequencies imposed on the test specimen 6 by the excitation, and (b) resonant natural frequencies of the test specimen either during or after application of the excitation. Such response may cause the failure of the test specimen 6 as a result of increased or decreased friction between parts, or general interference between parts, etc. There are various types of waveforms satisfying the shock response spectrum given by the shock response spectrum test. However, in MIL-STD-810F (which has been revised from MIL-STD-810E in January, 2000), a new effective shock duration Te is defined and certain conditions are required. Depending on the certain conditions, the shock response spectrum should be satisfied, the determined effective shock duration should be kept, and some compromise may be necessary on the condition of a waveform which can not satisfy the determined effective shock duration, namely, a waveform the duration of which is 20% longer than the effective shock duration. In other words, first, if the test specimen has no significant low frequency modal response, it is permissible to allow the low frequency portion of the SRS to fall out of tolerance, provided the high frequency portion begins at least one octave below the first natural mode frequency of the test item. In this case, the duration should be satisfied. Secondly, if the test specimen has significant low frequency modal response, it is permissible to allow the duration of the transient pulse to fall out of tolerance, provided the duration of the transient pulse does not exceed
      T    e    +      1          2      ⁢              f        min            (here, fmin is the lowest frequency in the SRS analysis). Moreover, if the duration of the transient pulse must exceed
      T    e    +      1          2      ⁢                          ⁢              f        min            in order to have the low frequency portion of the SRS within tolerance, a new shock procedure must be used. The effective shock duration can be considered as requisite to perform a sufficient test for interference between parts. However, because the conventional waveform synthesis methods have not satisfied the requirements of the determined effective shock duration, namely, because of the required waveform which has a duration much longer than the determined effective shock duration, it is disadvantageously impossible to test for damage or malfunction which may be caused by the interference between parts.
Furthermore, in the existing method for calculating a shock response time history in the single degree of freedom system by using a digital filter algorithm, there is no problem to calculate the shock response time history in the single degree of freedom system, provided that the sampling frequency is much higher than the natural frequency to be analyzed. However, its inverse applications to filter off the unwanted transient have been demonstrated not to be feasible because of instability problems.
Moreover, in the classic sawtooth or half-sine shock test which have pre-load and post-load pulses, there is no way to easily reduce peak velocities or peak displacements. As a result, it is impossible to perform the test in case of exceeding the shaker limits.